Off-road motorcycles are well-known forms of transportation that are built for use on rough terrain and riding them usually includes maneuvering through numerous curves and executing jumps. Thus, the responsiveness of the bike to the rider is critical to the bike having the proper performance. These motorcycles are built to be rugged and maneuverable.
Referring to FIGS. 1 and 2, conventional off-road motorcycles 10 generally include a frame 12, a front-suspension 13, steering assembly 14 that includes a front wheel 16, a fuel tank 18, a two-stroke engine 20, a seat 21, and a rear-suspension assembly 22 that includes a rear wheel 24. The motorcycle 10 further includes a brake assembly that includes front and rear brake cylinders 25.
The frame 12 shown is an aluminum frame and includes rearwardly divergent spars 26 that extend from a head tube 28 and an engine support assembly 30. The engine support assembly 30 is formed of tubular members that are fixed to the spars 26 and extend downwardly therefrom to form a space 32 for receiving the engine 20. The engine 20 is bolted to the engine support assembly 30 and spars 26 within the space 32.
Referring to FIG. 1, fuel tank 18 stores fuel to be burned by the engine 20. In general, conventional fuel tanks are positioned on the frame 12 between the head tube 28 and the seat 21. An air filter (not shown) is necessary to prevent dust and dirt from getting sucked into the engine 20 where it can cause harm.
Due to the location of the carburetor and air box 40 behind engine 20, the air filter is typically located below and behind the fuel tank 18. The fuel tank disclosed in U.S. Pat. No. 4,577,719 issued to Nomura et al. discloses a main fuel tank disposed above the engine, and a separate auxiliary tank formed below the seat and behind the engine. The auxiliary tank is connected to the main fuel tank by a pipe. The air filter is located beneath the fuel tank.
Inspection and replacement of the air filter is often necessary, because these types of motorcycles are ridden in the dirt and when air filter gets too dirty, it impairs engine performance. Easy access to the air filter is desired.
Several configurations afford improved access to the air filter. For example, U.S. Pat. No. 4,648,474 issued to Shinozaki et al. discloses a fuel tank including a forwardly facing U-shaped cavity. An air filter is located within the cavity of the fuel tank. The fuel tank is located above and on the exterior of the sides of the main frame. U.S. Pat. No. 4,653,762 issued to Nakamura et al. discloses a main fuel tank of a generally U-shape section straddling the main frame so that it downwardly extends toward an engine. The main fuel tank is formed with an open space in its rear upper portion that extends partially vertically through the tank. An air cleaner is disposed within the space, and covered with a detachable cover. Both of the aforementioned tank configurations are for road motorcycles not off-road motorcycles. Thus, these tanks are configured for larger fuel capacities than that desired for off-road motorcycles, and make off-road motorcycles more difficult to maneuver.
Referring to FIGS. 1 and 2, the engine 20 of a typical off-road motorcycle includes a cylinder 38 and a carburetor and air box 40, one of which is shown. The carburetor 40 mixes fuel with air, and this mixture is fed into the engine 20 and burned. The carburetor 40 is located behind the engine cylinder 38. A kick start 42 extends from the rear of one side of the engine 20 for starting the engine.
Most serious off-road motorcycles have two-stroke engines, which provide excellent power output. When installed, the cylinders 38 of two-stroke engines extend vertically and terminate so that they are spaced from the spars 26. As a result, when repair of the engine is necessary, the two-stroke engine is removed sideways from the frame.
Some off-road motorcycles have four-stroke engines, which are typically less powerful than two-stroke engines for the same displacement. However, four-stroke engines are significantly cleaner, more efficient and quieter. In the 1950's most motocross motorcycles had four-stroke engines. An early off-road, four-stroke, motorcycle was Rick Johnson's 1981-1984 Factory Team Bike built by Team Yamaha and Pro-Tec. Other four-stroke motorcycles are the Yamaha YZ400 and the ATK 350/605 and 600. All of these four-stroke bikes have steel frames and use carburetors for fuel/air mixture. In order to provide the same power as a two-stroke engine, the cylinder of the four-stroke engine must be significantly larger. Due to the size and weight of the four-stroke engine, the frame is made taller and the bike is heavier, which makes the bike more difficult to ride.
Steel off-road frames typically have a single spar extending from the head tube. The engine support assembly is fixed to the spar, and extends therebelow to form the space 32. In order to make the four-stroke engine fit in this type of frame, the spar is angled upward with respect to the head tube 28 so that the space 32 is large enough to accommodate the entire engine. This allows the engine to be removed sideways from the frame. However, enlarging the space 32 may cause the seat 21 to be higher than in the other configuration. Since a higher seat may make straddling the motorcycle while standing and maneuvering the motorcycle more difficult, this configuration is undesirable.
Due to the performance requirements of off-road motorcycles, they are generally equipped with a rugged rear suspension 44 capable of cushioning the rider from jarring. Typically, the rear suspension 44 of an off-road motorcycle consists of a swing arm 46 that is pivotally attached to the frame 12. The rear wheel 24 is rotatably mounted at the free ends of the swing arm 46. A shock 48 is connected between the swing arm 46 and the frame 12. Since the carburetor and air box 40 are located behind the engine 20, their presence greatly dictates the configuration of the shock 48 connection to the swing arm and main frame.
In order to obtain the desired performance, one arrangement provides a linkage 50 between the lower surface of the swing arm 46 and the shock 48. When the free ends of the swing arm 46 move toward the frame 12, the linkage moves and compresses the shock 48. Consequently, the shock biases the free ends of the swing arm 46 away from the frame 12. The linkage 50 allows the attachment point of the shock 48 to the frame 12 and the shock 48 itself to extend substantially vertically upward, such that it avoids the carburetor and air box 40. Since this arrangement requires the linkage 50, it is more complex than a direct connection of the shock to the swing arm. Therefore, it is undesirable.
Another arrangement directly connects the shock 48 to the swing arm 46. However in order to avoid the carburetors and air box 40, the shock must be off-set from the central plane of the motorcycle. The shock is coupled to either the right of left arm of the swing arm and, thus, loads and stresses that swing arm unevenly. Typically the swing arm must then be built to withstand these conditions.
Furthermore, this configuration requires the frame to be configured to compensate for the lack of symmetry of the shock mount and the shock loads. This will not maximize the maneuverability of the motorcycle. Another alternative is to use two shocks connected to each of the arms of the swing arm. However, this tends to make the motorcycle wide through the foot peg area and from a simplicity stand point, fewer parts would be more desirable.
Therefore, in order to overcome the shortcomings of the prior art, the object of the invention herein is to provide improved off-road motorcycle frame configuration and four-stroke engine.